The present invention relates to lasers and more particularly provides a new laser mechanism in which the upper lasing level is pumped by means of dissociative recombination.
Dissociative recombination is a phenomenon exhibited by certain molecular ions. Experimental evidence has revealed instances of dissociative recombination, occurring most readily in gas phase substances. Such gas phase substances may have numerous allotropes which can be ionized in many ways, including electrically, optically, and chemically. The ionic allotropes of a substance are not uniformly stable, leading to differences in the populations of the allotropes. For instance, for hydrogen the H.sub.3.sup.+ ion is more stable under usual conditions than the H.sub.2.sup.+ ion. As a result the numer of H.sub.3.sup.+ ions will be greater than H.sub.2.sup.+ ions in a given population. Sodium has a similar stable molecular ion, Na.sub.3.sup.+. Thus, in an ionized population there will be a predominance in the number of ions of a highly stable species in comparison to the number of ions of the less stable species.
In some cases, when the stable ions recombine with free electrons in the population, the ion dissociates. For example, upon recombination the Na.sub.3.sup.+ ion may dissociate into an Na.sub.2.sup.* species and a monatomic Na species. (The superscript asterisk * indicates that the molecule is in an excited electronic state.) The Na.sub.2.sup.* molecule is subject to further dissociation into two, separate monatomic Na entities following the emission of light in the violet wavelength region of the visible spectrum. The violet band of sodium was suggested as a possible laser candidate by F. G. Houtermans, Helv. Phys. Acta 33, (1960) 933, and J. P. Woerdman, "Laser-Excited Broad Band Violet Emission From A Sodium Molecule," Optics Communications 26, (1978) 216.
An atomic laser based on the dissociative recombination process was proposed in B. N. Smirnov, "A Molecular Ion Gas Laser," Soviet Physics-Doklady 13, (1969) 1148; L. Y. Efremenkova et al., "Ultra-Violet Laser Using the Lyman Transition," Soviet Physics-Doklady 17, (1972) 336; and A. V. Eletskii, et al., "Dissociative Recombination of Electrons and Molecular Ions," Sov. Phys. Usp. 25(1), (1982) 13. Recombination lasers generally were recently reviewed in E. L. Latush et al., "Metal Vapor Recombination Laser Research," Proc. Int'l. Conf. on Lasers '81 pp. 1121-1128, STS Press, McLean, Va.
Molecular lasers based on the dissociative recombination process have not previously been proposed.
In addition, the applicants have been involved in laser research for a number of years and have participated in generating the following publications in connection with such research:
A. Rajaei-Rizi et al., "New Optically Pumped Alkali Metal Dimer Lasers," Proc. Intl. Conf. on Lasers '81 pp. 447-449 (1981).
J. T. Bahns, et al., "New Alkali Metal Dimer Optically Pumped Lasers," Proc. Int. Conf. on Lasers '82 pp. 713-720 (1982).
J. T. Bahns, et al., "Optically Pumped Ring Laser Oscillation to Vibrational Levels Near Dissociation and To the Continuum of Na.sub.2," Applied Physics Letters 42, (1983) 336-338 (not admitted to be prior art).
Similarly, excimer lasers are known, such as those disclosed in U.S. Pat. No. 4,136,336 to Searles et al. and U.S. Pat. No. 4,249,143 to Eden. See also Ch. K. Rhodes, Editor, Excimer Lasers (Springer Verlag, New York 1979) and C. K. Rhodes, H. Egger and H. Pummer, Editors, Excimer Lasers-1983 (American Institute of Physics, New York 1983). The foregoing publications and patents are incorporated herein by reference.
Excimer lasers have the feature that a population of molecules in an excited state is formed and the molecules dissociate into atoms or less complex molecules following the emission of light. The coherent emission of such light from many molecules in the population is the laser emission. The emission of the photon is the result of a transition from a particular energy state of the molecule to a particular state of the atoms or less complex molecules. The energy state of the molecule is greater than that of the dissociated products, so during the transition, energy is given off in the form of a photon. The difference in energy determines the energy of the photon, and also its wavelength, in accordance with Planck's Law.
Excimer lasers have two valuable features. First, because the photon is emitted as the constituents of the molecule begin separating from one another, the reverse process, photon absorption, occurs very infrequently. That is, once the constituents are separated, they will not absorb a photon except in the unlikely event that they collide with one another and a photon simultaneously. Thus, unlike atomic lasers, the excimer laser output is almost never significantly reabsorbed by the lasing medium but instead is available for useful purposes. The reabsorption problem is known as "bottlenecking" and is successfully avoided in excimer lasers.
Second, because the upper state is a molecule, it has considerably more energy states than an atom. These arise from the vibrational and rotational quantum states of the molecule, states that an atom does not have. With this greater number of possible upper level energy states, a wide variety of transition energies is possible. In addition, when the constituents of the molecule separate, they can do so with a wide variety of kinetic energies, and thus a wide variety of transition energies. This variety of transition energies allows the excimer laser to be tuned to yield photons having a desired energy corresponding to a specific wavelength, determined by the precise difference in energy between the upper state of the molecule and the lower state of the dissociated products.
A tunable, efficient, high powered laser would be of great benefit in a number of applications. Among these are the pumping of existing dye lasers, photochemistry, photolithography, materials processing, surgery, and analytical fluorescence. However, to date the options available have been limited to tunable dye lasers for wavelengths in the violet band of visible light, which would be especially suitable for the above-mentioned applications. Accordingly, there is a need in the art for such a laser.